In the super PAC era of campaign politics, even Senate incumbents who appear safe in the summer can be on the ropes by the fall — and Democrats looking to protect their majority are already trying to ensure that none of their incumbents this cycle gets taken by an October surprise.

Democrats are defending a hefty 24 seats this year. And while their class of candidates has done quite well on paper in terms of fundraising, Members in second-tier races — where money isn’t pouring in as rapidly — privately have begun to express unease about having enough funds in the bank when the real money crunch comes this fall.

A popular and veteran lawmaker, Murray consistently held a 2- to 6-point advantage over her Republican opponent in 2010 before outside groups spent more than $8 million to try to defeat her, according to OpenSecrets.org. Though she won her race by almost4 points, she had been in a statistical dead heat in the polls by Election Day and had to spend $200,000 more than the$9.4 million that she raised to do it, according to campaign spending reports.

Now charged with protecting the Democratic seats, Murray is quick to remind her colleagues of her own race as she pushes them to keep building their campaign war chests.

“I remind them of what happened to me in the last election where I didn’t have an opponent until May of the election year, and it became one of the top five races because that’s what super PAC money does,” Murray said. “I think in this era, where we are seeing multinational corporations buying elections, big money, everybody has to work really hard.”

Of course, outside money cuts both ways, and Democrats have coalitions of supporters that will play in races on their behalf — especially big labor, which is likely to engage in Midwestern states such as Ohio or Michigan, where state legislatures are still debating labor laws and unemployment is high.

But Republican super PACs are likely to dominate, and they have already been successful in helping preferred conservative candidates in primaries.

According to campaign finance reports filed at the end of May, Club for Growth Action and FreedomWorks — two of the more influential conservative groups — are beginning to shift their focus to Congressional races. Club for Growth Action spent $2.2 million on House and Senate races in April, and FreedomWorks, a tea-party-promoting group, spent almost $400,000 on about a half-dozen races in that same time period.

Those disclosure reports revealed the groups have already targeted Democratic Sens. Claire McCaskill (Mo.), Jon Tester (Mont.), Bill Nelson (Fla.) and Debbie Stabenow (Mich.). Of that group, Nelson and Stabenow are widely considered to be safer and outside the top tier of targeted races.

Stabenow raised $1.5 million last quarter, and she has just shy of $7 million in cash on hand. Still, with a rocky economy and the potential for Republican momentum on the coattails of presumptive GOP presidential nominee Mitt Romney in his home state, Stabenow feels an urgency to raise even more.

“Oh, sure. Absolutely. I think that with the super PACs today, that’s something that’s in the backs of everybody’s minds,” Stabenow said. “There’s always that challenge, but I feel very fortunate so far because people have been very supportive so far.

“We’re all working hard [and] decisions will be made based on who has the [toughest] races,” Stabenow said of the distribution of campaign funds. “But I think the biggest thing is people are working hard and trying to protect ourselves.”

The most recent quarterly filings will not be available until next month. The beginning of this year, however, was a lucrative stretch for Democratic incumbents, particularly those in the tightest races. Democratic women have also been strong fundraisers, holding women’s campaign events and barnstorming the coasts for cash.

And in the Rust Belt states, Sens. Sherrod Brown (Ohio) and Bob Casey (Pa.) have $6.3 million and $5.3 million in cash on hand, respectively. Nelson has more than $9.5 million in the bank after raising $1.5 million last quarter.

Just how much money these candidates will need may depend on the strength of the national ticket in their respective states. Senate Democratic aides say they are trying to gird their candidates against huge swings based on the performance of President Barack Obama, for example, by identifying ticket-splitting voters. They pointed especially to Romney voters who might also vote for Tester, Nelson or Democratic Senate candidate Tim Kaine in Virginia. Aides noted it may be easier to find a Romney-Kaine voter than an Obama-George Allen supporter.

Some of these candidates, they suggested, could survive an Obama loss if it’s by a small margin but could lose on larger margins of national defeat.

In the end, however, everyone agrees that candidates regardless of party will have to fight for their victories, perhaps harder than they ever have in their careers and go back to the fundamentals of their first races.

“Everybody has to work hard,” Murray said. “And frankly I feel really good about people like Debbie — she goes home, she knows her state, she works hard, she’s doing everything she needs to do.”